The Composers were pieces of Forerunner technology that were created by the Forerunners in order to gain a foothold against the Flood in the Forerunner-Flood War. They are capable of bridging organic life and digital technology together. One was used by the Ur-Didact on ancient Humans, adding to his legion of digitized beings called the Prometheans. The Composers were originally intended to allow the Forerunners to become immortal. The Didact pursues this piece of technology in Halo 4.

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The Composer was intended to bridge the physical and digital realms, thereby making the Forerunners immortal. However, the device was flawed: the stored personalities fragmented and attempts to restore them to biological form failed.

As the Forerunner-Flood War raged, the Didact chose to use the Composer to transform his Promethean warriors into AI constructs, a process he refers to as "Composing." The Didact believed that the Flood could be defeated this way, since they could only infect living tissue (the Prometheans being both digital and inorganic). However, the lack of moral values in the resulting Prometheans caused the Forerunners to abandon the project. This was likely due to the fact that because Humans were seen as being the next race to uphold the Mantle, they had to have moral values. The Didact ignored this however and continued using it. But as the Prometheans lacked the sufficient numbers to turn back the tide, the Didact chose to Compose a population of Humans settled on Omega Halo. To the Didact, it would be like killing two birds with one stone; he would punish Humanity for their past crimes while using them to defeat the Flood. Such callous and amoral means upset The Librarian, who confiscated the Composer from the Didact and hid it on Installation 03.

The UNSC discovered a Composer on Installation 03 prior to 2557 and had the UNSC Infinity move it to the adjacent Ivanoff Research Station for study, a process which took 3 months according to Sandra Tillson. While researching the Composer, several members of the team were affected by it and reduced to ash, prompting the team to proceed more carefully. Sensor data found after this incident gave coordinates that lead to Requiem.

In 2557, the Didact escaped from Requiem and attacked Ivanoff Station with his Prometheans and a Covenant remnant group. He successfully captured the Composer and fired it at Ivanoff Station, instantly killing all on board except for the Master Chief, who was immune to its effects due to the Librarian's genetic manipulation. The Didact brought the Composer to Earth to destroy humanity and was even able to fire it at Earth. The city of New Phoenix was hit, and every human in it was instantly "composed." Master Chief was able to board Mantle's Approach, plant a Pulse Grenade on the Didact's chest (as multiple versions of Cortana were binding him down via Hard Light manifestation) then destroy the Composer and the flagship with a HAVOK Tactical Nuclear Weapon.

The Composer fires an orange beam to harvest the "essence" of an organism, and convert it into a Promethean AI matrix. It is not exactly known how this works, but in a personal log, Catherine Halsey describes this device as similar to the UNSC AI Matrix compiler that she developed.

When used on an organism, it painfully burns away the body layer by layer until that organism's "essence" is collected, leaving a pile of ash with a blue particle effect.

John-117 and the Ur-Didact have a genetic ability to resist the Composer's effects; the former via a genetic mutation conducted by the Librarian's contact essence on Requiem, and the latter through mutations done upon himself in an attempt to become immune to the Flood.

All Forerunner Ordnance weapons have a similar disintegration effect as the Composer.